Conventionally, a three-way catalyst (catalytic unit for purifying an exhaust gas) is disposed in an exhaust passage of an internal combustion engine in order to purify the exhaust gas discharged from the engine. As is well known, the three-way catalyst has an “oxygen storage function” for storing oxygen flowing into the three-way catalyst and releasing the stored oxygen.
Hereinafter, the three-way catalyst is simply referred to as a “catalyst”, the gas flowing into the catalyst is referred to as a “catalyst inflow gas”, and the gas flowing out from the catalyst is referred to as a “catalyst outflow gas.” Further, an air-fuel ratio smaller than a stoichiometric air-fuel ratio is referred to as a “rich air-fuel ratio”, an air-fuel ratio larger than a stoichiometric air-fuel ratio is referred to as a “lean air-fuel ratio”, and an air-fuel ratio of a mixture supplied to the engine is referred to as an “air-fuel ratio of the engine.”
One of conventional air-fuel ratio control apparatuses (hereinafter, referred to as a “conventional apparatus”) comprises a downstream air-fuel ratio sensor. The downstream air-fuel ratio sensor is disposed downstream of the catalyst in the exhaust passage of the engine. The conventional apparatus controls the air-fuel ratio of the engine (and thus, an air-fuel ratio of the catalyst inflow gas) in such a manner that an output value of the downstream air-fuel ratio sensor coincides with a value corresponding to the stoichiometric air-fuel ratio (refer to, for example, Patent Literature No. 1)
More specifically, the conventional apparatus controls the air-fuel ratio of the engine in such a manner that the air-fuel ratio of the catalyst inflow gas becomes the lean air-fuel ratio when the output value of the downstream air-fuel ratio sensor becomes a value corresponding to the rich air-fuel ratio. In other words, when the output value of the downstream air-fuel ratio sensor has become the value corresponding to the rich air-fuel ratio, the conventional apparatus determines that a status/condition of the catalyst has become an “oxygen shortage state (rich state)”, and therefore, determines that a lean request has occurred, so that it controls the air-fuel ratio of the engine to be the lean air-fuel ratio.
Further, the conventional apparatus controls the air-fuel ratio of the engine in such a manner that the air-fuel ratio of the catalyst inflow gas becomes the rich air-fuel ratio when the output value of the downstream air-fuel ratio sensor becomes a value corresponding to the lean air-fuel ratio. In other words, when the output value of the downstream air-fuel ratio sensor has become the value corresponding to the lean air-fuel ratio, the conventional apparatus determines that the status/condition of the catalyst has become an “oxygen excessive state (lean state)”, and therefore, determines that a rich request has occurred, so that it controls the air-fuel ratio of the engine to be the rich air-fuel ratio. Accordingly, the apparatus can shorten a period in which the state of the catalyst is either “the oxygen shortage state or the oxygen excessive state”, and thus, can shorten a period in which an emission is unfavorable.